Arrangements are known for generating and inserting signal test patterns or sequences to evaluate the performance of transmission facilities. Such prior known arrangements typically inserted the test pattern or sequence at some predetermined place in a particular data stream at some uniform rate.
In more recently developed transmission systems, for example, asynchronous transfer mode (ATM) systems, it is necessary to evaluate the performance of so-called logical channels, each of which comprises an individual packetized data stream. A plurality of such logical channels form a combined data stream which is typically transmitted over a single transmission medium. The ATM transmission is on a so-called cell basis, wherein each cell comprises a fixed length packet. If there are no data to fill the fixed length packet, so-called idle cells are inserted. Additionally, the individual packets or cells of any particular logical channel in the combined data stream are not in any prearranged or preallocated position relative to individual packets or cells in other logical channels. They are only in a predetermined sequence relative to the other packets, i.e., cells, in their individual logical channel. Consequently, there is no fixed bandwidth to insert a test pattern or sequence as in the prior known arrangements. The test pattern, i.e., the test cell, in an ATM system can only be inserted when there are idle cells or when there are no useful cells being inputted, both hereinafter referred to as test cell insertion opportunity intervals. Such test cell insertion opportunity intervals typically occur non-uniformly. However, it is desirable to insert the desired test cell at some predetermined average rate. Because of the non-uniform nature of opportunities for inserting the test cell in ATM systems, prior arrangements do not operate satisfactorily.